House Republicans’ proposal to rewrite federal health-care law would more than reverse the gains the Affordable Care Act has made in the number of Americans with health insurance, while curbing the federal deficit, according to a widely anticipated forecast by congressional analysts.

The analysis, released late Monday afternoon by the Congressional Budget Office, predicts that 24 million fewer people would have coverage a decade from now than if the Affordable Care Act remains intact, nearly doubling the share of Americans who are uninsured from 10 percent to 19 percent. The office projects the number of uninsured people would jump 14 million after the first year

But the GOP legislation, which has been speeding through House committees since it was introduced a week ago, would lower the deficit by $337 billion during that time, primarily by lessening spending on Medicaid and government aid for people buying health plans on their own.

The report predicted that premiums would be 15 percent to 20 percent higher in the first year compared with those under the Affordable Care Act but 10 percent lower on average after 2026. By and large, older Americans would pay “substantially” more and younger Americans less.

The 37-page report provides the most tangible evidence to date of the human and fiscal impact of the House GOP’s American Health Care Act. It also undermines President Trump’s pledge that no Americans would lose coverage under a Republican remake of the Affordable Care Act, which was enacted by a Democratic Congress in 2010.

The report’s arrival produced starkly different tacks from the White House and Capitol Hill — with top aides to the president immediately seeking to discredit it while the House’s Republican leaders praised the report for reinforcing their argument that the plan curbs federal spending and gives Americans the freedom to be insured or not — their choice.

House Speaker Paul D. Ryan (R-Wis.), meanwhile, said in a Fox News interview that the report “exceeded” his expectations, and he jumped on its prediction of a smaller deficit to try to assuage the chamber’s most conservative members, many of whom oppose the idea of new tax credits to help some Americans buy coverage on their own.

Declaring that the plans would usher in “the most fundamental entitlement reform in a generation,” Ryan said the legislation “is about giving people more choices and better access to a plan they want and can afford. When people have more choices, costs go down. That’s what this report shows.”

Despite that sales pitch, early signs emerged Monday night that the Congressional Budget Office report was not helping to solidify GOP support. Rep. Rob Wittman (R-Va.) announced he would oppose the bill.

“I do believe that we can enact meaningful health care reforms that put the patient and health care provider back at the center of our health care system, but this bill is not the right answer,” he said in a Facebook post.

Wittman’s stance could represent a new front of dissent among House Republicans. A six-term member who leads a House Armed Services subcommittee and represents a district that favored Trump by 12 percentage points, Wittman is neither a hard-right firebrand nor a wary moderate from a Medicaid expansion state. Rather, he is the sort of mainstream conservative that Ryan is counting on to toe the party line and pass the bill.

The release of the analysis marks the beginning of a new phase in the debate over the week-old health-care bill, which is moving through the House despite opposition from many Republicans, Democrats and major sectors of the U.S. health-care industry.

Democrats used the report’s findings to continue excoriating the House GOP plan. “The CBO score shows just how empty the president’s promises, that everyone will be covered and costs will go down, have been,” said Senate Minority Leader Charles E. Schumer (D-N.Y.). “This should be a looming stop sign for the Republicans’ repeal effort.”

The analysis predicts that the number of people without health coverage would rise to 52 million by 2026, compared with 28 million if the Affordable Care Act remains intact. That erosion would mean that about 1 in 5 U.S. residents would be uninsured by 2026 — compared to 1 in 10 uninsured now and 1 in 6 who were uninsured before the Affordable Care Act was enacted.

The reduction in the number of insured people would result from three factors. A provision rescinding the penalty imposed on the uninsured could prompt many Americans to drop their health plans. After that, tax credits that are less generous than current subsidies could make insurance unaffordable to more people. Finally, some states may undo the expansion of their Medicaid programs.

The conservative House Freedom Caucus did not immediately respond to the report. Moderate Republicans expressed concerns about the number of people who would lose coverage, foreshadowing possible problems for the legislation if it reaches the Senate.

“These kinds of estimates are going to cause revisions in the bill, almost certainly,” said Sen. Susan Collins (R-Maine).

“I don’t think that the bill that is being considered now is the bill that ultimately will be the one that we vote on in the Senate.”

In its current form, the House GOP proposal would administer Medicaid by giving each state a fixed amount of funding per person in the program rather than covering a fixed percentage of its Medicaid costs, no matter how high. The plan would also replace the Affordable Care Act’s federal insurance subsidies with age- and income-based tax credits, which would involve considerably less spending, the report shows.

While the deficit would be lower, the legislation also would reduce federal revenue by $592 billion by 2026 by repealing several taxes that the Affordable Care Act created to help pay for more people to get insurance — notably taxes on high-income Americans, hospitals and health insurers.

“They are implementing the biggest transfer of wealth in our history,” House Minority Leader Nancy Pelosi (D-Calif.) told reporters Monday. “In terms of insurance coverage, it’s immoral. In terms of giving money to the rich at the expense of working families, it is indecent and wrong.”

The White House has spent the past week engaged in a charm offensive aimed at bringing conservatives on board, as well as an effort to discredit the Congressional Budget Office before it released numbers.

The Affordable Care Act has increased coverage by 20 million to 22 million — almost half of those through the insurance markets the law created for people who cannot get affordable coverage through a job, and the rest through an expansion of Medicaid in 31 states and the District of Columbia.

But Cornyn noted that, by eliminating the penalty for violating the Affordable Care Act’s requirement that most Americans carry insurance, “some people are going to make the decision not to buy it.”

The elimination of that penalty would account for the immediate increase in the uninsured.

The estimates projected a significant drop in Medicaid enrollment. Next year, the forecast says, about 5 million fewer people would be on Medicaid. By 2026, the program’s rolls would shrink by nearly 15 million — almost 1 in 4 of the 68 million currently in the program.

The Congressional Budget Office also predicted substantial disparities in the effect the legislation would have on insurance premiums for younger versus older consumers.

If the GOP plan is enacted, a 21-year-old making $68,200 would pay an average of $1,450 for a year’s worth of insurance premiums after the new tax credits, compared with $5,100 under the current law.

On the other hand, the cost of a year’s worth of premiums would stay about the same for a 64-year-old at the same income level. For a 64-year-old making $26,500, the cost would rise sharply, from $1,700 to $14,600.

“I’m not an attack person — you know that,” he said. “I don’t just attack because you’re on the other side of the aisle. But how can you look at yourself and say, ‘Okay, I’ll help the person who needs help the least, the wealthiest people, with more tax cuts, because I’m going to be taking away from the elderly population?’ ”

The analysis also forecast a reduction in the number of Americans who get insurance through their employers, in part because the new tax credits would be available to people with higher incomes than with the Affordable Care Act’s subsidies. Some employers would also drop coverage, the Congressional Budget Office projected.

“President Trump said that he wants as many people covered as under Obamacare,” Cassidy said. “He said that health care should be affordable. If there’s 14 million people losing insurance, of course it’s concerning. I try to avoid hyperbole and adjectives, but it’s concerning.”

Abby Phillip, Sean Sullivan and David Weigel contributed to this report.